Global Mean Ocean Temperature Research Articles

Enhanced ocean heat storage efficiency during the last deglaciation

Proxy reconstructions suggest that increasing global mean sea surface temperature (GMSST) during the last deglaciation was accompanied by a comparable or greater increase in global mean ocean temperature (GMOT), corresponding to a large heat storage efficiency (HSE; ∆GMOT/∆GMSST). An increased GMOT is commonly attributed to surface warming at sites of deepwater formation, but winter sea ice covered much of these source areas during the last deglaciation, which would imply an HSE much less than 1. Here, we use climate model simulations and proxy-based reconstructions of ocean temperature changes to show that an increased deglacial HSE is achieved by warming of intermediate-depth waters forced by mid-latitude surface warming in response to greenhouse gas and ice sheet forcing as well as by reduced Atlantic meridional overturning circulation associated with meltwater forcing. These results, which highlight the role of surface warming and oceanic circulation changes, have implications for our understanding of long-term ocean heat storage change.

Snapshots of mean ocean temperature over the last 700 000 years using noble gases in the EPICA Dome C ice core

Abstract. Together with the latent heat stored in glacial ice sheets, the ocean heat uptake carries the lion's share of glacial–interglacial changes in the planetary heat content, but little direct information on the global mean ocean temperature (MOT) is available to constrain the ocean temperature response to glacial–interglacial climate perturbations. Using ratios of noble gases and molecular nitrogen trapped in the Antarctic EPICA Dome C ice core, we are able to reconstruct MOT for peak glacial and interglacial conditions during the last 700 000 years and explore the differences between these extrema. To this end, we have to correct the noble gas ratios for gas transport effects in the firn column and gas loss fractionation processes of the samples after ice core retrieval using the full elemental matrix of N2, Ar, Kr, and Xe in the ice and their individual isotopic ratios. The reconstructed MOT in peak glacials is consistently about 3.3 ± 0.4 ∘C cooler compared to the Holocene. Lukewarm interglacials before the Mid-Brunhes Event 450 kyr ago are characterized by 1.6 ± 0.4 ∘C lower MOT than the Holocene; thus, glacial–interglacial amplitudes were only about 50 % of those after the Mid-Brunhes Event, in line with the reduced radiative forcing by lower greenhouse gas concentrations and their Earth system feedbacks. Moreover, we find significantly increased MOTs at the onset of Marine Isotope Stage 5.5 and 9.3, which are coeval with CO2 and CH4 overshoots at that time. We link these CO2 and CH4 overshoots to a resumption of the Atlantic Meridional Overturning Circulation, which is also the starting point of the release of heat previously accumulated in the ocean during times of reduced overturning.

A new DRP-4DVar-based coupled data assimilation system for decadal predictions using a fast online localization technique

A new coupled data assimilation (CDA) system based on dimension-reduced projection four-dimensional variational data assimilation (DRP-4DVar) for decadal predictions is developed and applied to a fully coupled model FGOALS-g2, which applies a fast online localization technique. The improved CDA system can assimilate more observational information than the previous system, with a larger reduction in the observational cost function and smaller biases and root mean square errors (RMSEs) in global mean ocean temperature and salinity. The assimilated climate variabilities in the Pacific and Atlantic, such as El Ni$$\tilde{\text{n}}$$o-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), North Pacific index (NPI), Pacific-North American teleconnection (PNA) and Atlantic Multi-decadal Oscillation (AMO), are generally better reproduced by the new CDA system compared to the previous system and the uninitialized simulation. The Atlantic meridional overturning circulation (AMOC) is better described than the previous system. After partially restoring the climatology of the initial condition to that of the model to relieve the initial shock, ten-member decadal prediction experiments are started each year from 1961 to 1996. Higher decadal prediction skills of near-surface air temperature anomalies over the North Pacific, the Atlantic, and the Indian Oceans and the Eurasia Continent are achieved by the new system compared to those obtained by the previous CDA system, persistence and the uninitialized simulation.

Climate and marine biogeochemistry during the Holocene from transient model simulations

Abstract. Climate and marine biogeochemistry changes over the Holocene are investigated based on transient global climate and biogeochemistry model simulations over the last 9500 years. The simulations are forced by accelerated and non-accelerated orbital parameters, respectively, and atmospheric pCO2, CH4, and N2O. The analysis focusses on key climatic parameters of relevance to the marine biogeochemistry, and on the physical and biogeochemical processes that drive atmosphere–ocean carbon fluxes and changes in the oxygen minimum zones (OMZs). The simulated global mean ocean temperature is characterized by a mid-Holocene cooling and a late Holocene warming, a common feature among Holocene climate simulations which, however, contradicts a proxy-derived mid-Holocene climate optimum. As the most significant result, and only in the non-accelerated simulation, we find a substantial increase in volume of the OMZ in the eastern equatorial Pacific (EEP) continuing into the late Holocene. The concurrent increase in apparent oxygen utilization (AOU) and age of the water mass within the EEP OMZ can be attributed to a weakening of the deep northward inflow into the Pacific. This results in a large-scale mid-to-late Holocene increase in AOU in most of the Pacific and hence the source regions of the EEP OMZ waters. The simulated expansion of the EEP OMZ raises the question of whether the deoxygenation that has been observed over the last 5 decades could be a – perhaps accelerated – continuation of an orbitally driven decline in oxygen. Changes in global mean biological production and export of detritus remain of the order of 10 %, with generally lower values in the mid-Holocene. The simulated atmosphere–ocean CO2 flux would result in atmospheric pCO2 changes of similar magnitudes to those observed for the Holocene, but with different timing. More technically, as the increase in EEP OMZ volume can only be simulated with the non-accelerated model simulation, non-accelerated model simulations are required for an analysis of the marine biogeochemistry in the Holocene. Notably, the long control experiment also displays similar magnitude variability to the transient experiment for some parameters. This indicates that also long control runs are required when investigating Holocene climate and marine biogeochemistry, and that some of the Holocene variations could be attributed to internal variability of the atmosphere–ocean system.

Noble gases as proxies of mean ocean temperature: sensitivity studies using a climate model of reduced complexity

Past global mean ocean temperature may be reconstructed from measurements of atmospheric noble gas concentrations in ice core bubbles. Assuming conservation of noble gases in the atmosphere-ocean system, the total concentration within the ocean mostly depends on solubility which itself is temperature dependent. Therefore, the colder the ocean, the more gas can be dissolved and the less remains in the atmosphere. Here, the characteristics of this novel paleoclimatic proxy are explored by implementing krypton, xenon, argon, and N 2 into a reduced-complexity climate model. The relationship between noble gas concentrations and global mean ocean temperature is investigated and their sensitivities to changes in ocean volume, ocean salinity, sea-level pressure and geothermal heat flux are quantified. We conclude that atmospheric noble gas concentrations are suitable proxies of global mean ocean temperature. Changes in ocean volume need to be considered when reconstructing ocean temperatures from noble gases. Calibration curves are provided to translate ice-core measurements of krypton, xenon, and argon into a global mean ocean temperature change. Simulated noble gas-to-nitrogen ratios for the last glacial maximum are δKr atm = −1.10‰, δXe atm = −3.25‰, and δAr atm = −0.29‰. The uncertainty of the krypton calibration curve due to uncertainties of the ocean saturation concentrations is estimated to be ±0.3 °C. An additional ±0.3 °C uncertainty must be added for the last deglaciation and up to ±0.4 °C for earlier transitions due to age-scale uncertainties in the sea-level reconstructions. Finally, the fingerprint of idealized Dansgaard-Oeschger events in the atmospheric krypton-to-nitrogen ratio is presented. A δKr atm change of up to 0.34‰ is simulated for a 2 kyr Dansgaard-Oeschger event, and a change of up to 0.48‰ is simulated for a 4 kyr event.

Emulating AOGCM results using simple climate models

Three simple climate models (SCMs) are calibrated using simulations from atmosphere ocean general circulation models (AOGCMs). In addition to using two conventional SCMs, results from a third simpler model developed specifically for this study are obtained. An easy to implement and comprehensive iterative procedure is applied that optimises the SCM emulation of global-mean surface temperature and total ocean heat content, and, if available in the SCM, of surface temperature over land, over the ocean and in both hemispheres, and of the global-mean ocean temperature profile. The method gives best-fit estimates as well as uncertainty intervals for the different SCM parameters. For the calibration, AOGCM simulations with two different types of forcing scenarios are used: pulse forcing simulations performed with 2 AOGCMs and gradually changing forcing simulations from 15 AOGCMs obtained within the framework of the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. The method is found to work well. For all possible combinations of SCMs and AOGCMs the emulation of AOGCM results could be improved. The obtained SCM parameters depend both on the AOGCM data and the type of forcing scenario. SCMs with a poor representation of the atmosphere thermal inertia are better able to emulate AOGCM results from gradually changing forcing than from pulse forcing simulations. Correct simultaneous emulation of both atmospheric temperatures and the ocean temperature profile by the SCMs strongly depends on the representation of the temperature gradient between the atmosphere and the mixed layer. Introducing climate sensitivities that are dependent on the forcing mechanism in the SCMs allows the emulation of AOGCM responses to carbon dioxide and solar insolation forcings equally well. Also, some SCM parameters are found to be very insensitive to the fitting, and the reduction of their uncertainty through the fitting procedure is only marginal, while other parameters change considerably. The very simple SCM is found to reproduce the AOGCM results as well as the other two comparably more sophisticated SCMs.

Recovery of thermohaline circulation under CO 2 stabilization and overshoot scenarios

In this study we examine the behavior of the thermohaline circulation, as simulated by the Community Climate System Model version 3 (CCSM3), for several centuries following CO 2 stabilization for the SRES B1 and A1B scenarios and for an “overshoot” scenario in which CO 2 levels temporarily reach the same level as in the A1B scenario before declining to an ultimate stabilization level that is identical to the B1 case. While we find no evidence for irreversible changes of the thermohaline circulation in the overshoot experiment, the interplay of the different timescales of the temperature response of the surface and interior ocean does lead to a number of differences in the long-term response of the ocean between it and the B1 stabilization scenario where the same GHG levels are approached by different paths. The stronger initial warming and its slow penetration into the deeper ocean, followed by a transient surface cooling in the overshoot scenario leads to lower static stability, deeper mixing, and a more rapid recovery of the thermohaline circulation than in the B1 stabilization scenario. While the overshoot scenario recovers surface conditions (e.g. SST, sea ice extent) very similar to the B1 scenario shortly after reaching the same GHG levels, the additional accumulation of heat in the interior ocean during the period of higher forcing causes the global mean ocean temperature and steric sea level to remain higher than in the B1 stabilization scenario for at least another several centuries.

A first-order global model of late Cenozoic climatic change II. Further analysis based on a simplification of CO2 dynamics

A model developed recently for the long-term variations of global ice mass, carbon dioxide, and mean ocean temperature through the late Cenozoic is simplified by hypothesizing a new equation for the CO2 variations containing one less adjustable parameter, but retaining the essential physical content of the previous equation (including nonlinearity and the potential for instability). By assuming plausible time constants for the glacial ice mass and global mean ocean temperature, and setting the values of six adjustable parameters (rate constants), a solution for the last 5 My is obtained displaying many of the features observed over this period, including the transition to the near-100 ky major ice-age oscillations of the late Pleistocene. In obtaining this solution it is also assumed that variations in tectonic forcing lead to a reduction of the equilibrium CO2 concentration (perhaps due to increased weathering of rapidly uplifted mountain ranges over this period). As a consequence of this CO2 reduction, the model dynamical system can bifurcate to a free oscillatory ice-age regime that is under the “pacemaker” influence of earthorbital (Milankovitch) forcing. Expanded discussions are given of the surface temperature variations accompanying the evolution of ice, CO2, and ocean temperature, and of the bifurcation properties of the model from both mathematical and physical viewpoints.